The present invention relates to mechanical pumps. More particularly, the invention relates to piston-array pumps suitable for use in analytical instrumentation.
There are many different categories of pumps commonly used in instrumentation. Examples include such types as: centrifugal, diaphragm, gear, peristaltic, piston, and syringe. This evaluation will cover the mode of operation and advantages and disadvantages of each. In particular, the relevance of the characteristics of each type to FIA (Flow-Injection Analysis), SIA (Sequential Injection Analysis), and HPLC (High Pressure Liquid Chromatography) will be examined. These methodologies will hereafter be referred to as "Flow-Based Analytical Techniques".
Centrifugal pumps are typically not used directly in the analytical portions of instrumentation, as they are incapable of delivering sufficiently precise control of flow. They do, however, find use as sample loop circulation devices. They have the disadvantages of not being self-priming, are unidirectional in their pumping action, and cannot pump against much back pressure. They have the advantages of not requiring the use of check valves to regulate cycle direction, and of providing pulseless flow output. Use of this type of pump to provide micro-flows is highly unlikely, and they are typically not used in Flow-Based Analytical Techniques.
Diaphragm pumps have several advantages over centrifugal. They are self-priming, and they will pump against more back pressure. Again, precise flow control is not possible, pumping action is unidirectional, they require check valves to control flow direction, and they deliver highly pulsating flows. These pumps, like centrifugal pumps, are usually used only to drive sample loop flows. Use of this type of pump to provide micro-flows is unlikely, and this type, too, is not usable in Flow-Based Analytical Techniques.
Gear pumps have advantages over both centrifugal and diaphragm pumps. Like diaphragm pumps, they are self-priming, can pump against more back pressure than centrifugal pumps, and can provide higher output pressures than centrifugal pumps. Like centrifugal pumps, their output is pulseless. Unlike centrifugal or diaphragm units, gear pumps can pump either backwards or forwards, and require no check valves. Precise control of flow rate is difficult, as the gearing surfaces are difficult to seal against "slip leakage", and precise flow control becomes problematical when pumping against high back pressure. Gear pumps are also more vulnerable to damage from particulate matter, which causes mechanical wear and increases the "slip leakage" problem. One gear pump version (an experimental design produced by Transcience) purports to eliminate slip leakage by using elastomeric gears. The limiting problem with the Transcience pump is likely to be longevity--the elastomeric gearing will probably not deliver long time intervals without failure, due to rapid wear of the gears. This type of pump (Transcience) purports to deliver micro-flow capability. The Transcience pump is intended for use in the FIA/SIA market niche. It will not be usable in HPLC due to low pressure limitations.
Peristaltic pumps are self-priming, can pump against more back pressure than centrifugal pumps, require no check valves to regulate flow direction, and can pump bidirectionally. They have the significant advantage of being able to gang multiple pumping heads on a single drive unit, and delivering all flows "in phase". The flows delivered by peristaltic pumps are "pulsating"--to a lesser extent than piston pumps, but still significantly. The really significant problem with peristaltic devices is the rapid failure of the elastomeric pump tubing used due to "plastic fatigue" from mechanical wear. Micro-flow delivery from this pump type is problematic due to the very small i.d. pump tubing required. This is the current pump of choice for laboratory FIA, it can also be used for SIA, but not for HPLC due to low pressure limits. Use of peristaltic pumps in instrumentation for process control is unacceptable due to the high maintenance requirements caused by pump tube wear.
Piston pumps are self-priming, can pump against the most back pressure of any pump type available, and can deliver very precise and easily regulated flow velocities. They provide significantly pulsing flow, pump unidirectionally, and most types require the incorporation of check valves. This type of pump can easily be designed to deliver micro-flow capability, and can be configured to drive multiple heads with a single drive unit. This type of pump is suitable for FIA, provided the detector can tolerate some flow pulsation. It cannot be used in SIA due to its unidirectional pumping action. It is the pump of choice for most HPLC work, and in instrumentation for process control, due to its high reliability. ELDEX is a manufacturer of a unit typical of this type of pump. Piston pumps for instrumentation are usually designed primarily for HPLC, which requires pressure capabilities significantly higher than FIA or SIA (1000-5000 psi), and thus makes them relatively expensive, which expense increases rapidly if multiple streams must be pumped.
A unique subset of piston pumps is produced by FMI, Inc. This pump utilizes a special pumping cycle incorporating a piston that simultaneously reciprocates (providing pumping action), and rotates (providing valving action). This type of pump is self-priming, can pump against significant back pressure (but less than pumps designed specifically for HPLC), has no check valves, can deliver bi-directional pumping action, and allows very precise and easily regulated flow velocities. The pumping action is inherently pulsating. These pumps can easily deliver micro-flow capability, and can be configured to drive multiple heads with a single drive unit. They are suitable for FIA, and as they are bi-directional, can be used in SIA (again, with the limitation that the detector tolerate flow pulsation). They are not suitable for HPLC due to low output delivery pressure.
Syringe pumps are essentially very large piston pumps. They are self-priming, can deliver very high pressures, and, as long as their initial fill charge lasts, can deliver pulseless flow rates. They can easily be designed to deliver micro-flows. However, their cycle does require a long refill cycle once the fill charge is exhausted, which is a potential problem (and one soluble by using duplex syringe pumps). They also require some sort of external valving arrangement to control the fill/pump cycle. This type of pump (especially if used in a duplex configuration) is suitable for FIA, SIA, and HPLC. Practical experience has shown glass-barreled syringe pumps to be fragile, and a need to exercise care with the syringe barrel and plunger.
Prior art relating specifically to piston-array pumps include the following patents.
U.S. Pat. No. 2,518,619 to Huber discloses a cylindrical ring valve for multicylinder pumps.
U.S. Pat. No. 3,981,630 to Leduc et al. discloses a swash-plate pump wherein a plurality of pistons bear against the swash plate and are given a reciprocating movement when the plate is rotated by a drive shaft.
U.S. Pat. No. 4,880,361 to Ikeda et al. discloses a multi-piston swash-plate compressor for an air-conditioning system used in a motor vehicle. The compressor has combined cylindrical blocks closed at both axial end faces thereof by front and rear housings.
U.S. Pat. No. 5,009,574 to Ikeda et al. discloses a swash-plate compressor having a pair of axially combined front and rear cylindrical blocks forming therein a plurality of cylindrical bores, a swash-plate chamber, and an oil chamber in which lubricating oil is stored to be stirred by a swash plate rotatably received in the swash-plate chamber, a drive shaft centrally and rotatably mounted in the combined cylindrical blocks to effect rotation of the swash plate, a plurality of reciprocatory double-headed pistons slidably fitted in the bores and operatively engaged with the swash plate via shoe members to be reciprocated by the rotation of the swash plate, a pair of thrust bearings axially supporting the swash plate, and front and rear housings having suction chambers for the refrigerant gas after compression. The front housing has a shaft-sealing chamber formed therein and separated from the suction chamber thereof to define an intermediate pressure chamber between the high-pressure swash-plate chamber and the low-pressure suction chamber of the front housing, and a thin fluid passageway interconnecting the shaft-sealing chamber with the suction chamber. The intermediate pressure chamber and the thin fluid passageway prevent evacuation of the lubricating oil from the swash-plate chamber to the suction chamber even during the rotation of the compressor at a high speed, to thereby promote a lubrication of the thrust bearings, the shoes, and the swash plate.
U.S. Pat. No. 4,095,921 to Hiraga et al. discloses a multi-cylinder compressor suitable for use in a vehicular air-conditioning system. The compressor includes a pair of axially-spaced cylindrical blocks for receiving a refrigerant fluid therein for compression. First and second sets of pistons are respectively reciprocated within the front and rear cylinders, respectively, by first and second sets of rods of different axial lengths. A lubricating system for the compressor includes a flapper element located near the oil hole, to direct oil to the shaft seal for both clockwise and counterclockwise rotation of the compressor.
U.S. Pat. No. 2,475,350 to Capsek discloses a fuel-injection pump comprising a series of elementary parallel piston pumps arranged circularly about the longitudinal axis of the assembly.
U.S. Pat. No. 4,360,321 to Copp, Jr. et al. discloses a multi-cylinder refrigerant compressor having double-ended pistons operating in aligned cylinder bores of a cylinder block to discharge refrigerant from the opposite ends thereof to discharge chambers formed in opposite ends of the compressor. A muffler arrangement is completely formed within the compressor, and comprises a separate attenuation chamber ported at one of two opposing ends thereof directly to each discharge chamber. Each attenuation chamber is formed within and as an integral part of the cylinder block between two adjacent cylinder walls thereof, and an elongated attenuation passage directly connects the attenuation chambers at their other ends. The attenuation chamber is also formed in and as an integral part of the cylinder block, and extends between the two adjacent cylinder walls. The volumes of the attenuation chambers are substantially equal, and the length of the attenuation passage is substantially longer than the corresponding longitudinal dimension of the attenuation chambers in order to attenuate the refrigerant discharge pulses admitted to the discharge chambers to an acceptable output level totally within the compressor.
None of these prior-art pumps is capable of providing the precise, pulseless, and reversible flow of fluid action necessary for chemical analysis. A need therefore exists for a pump which provides such capability. Such a pump, which would have great versatility, is provided by the present invention.